With advances in technology and more demand of increased data, customers need 10 Gb/s coarse wavelength division multiplexing (CWDM) optical transceivers that consume low power to fit into existing wireless communications systems that have been deployed in field. The term “CWDM” as used herein refers to a coarse wavelength division multiplexing technology. It is a method of combining multiple signals on laser beams at various wavelengths for transmission over optical fiber, and a number of channels of CWDM technology is fewer than that of dense wavelength division multiplexing (DWDM) technology, but more than standard wavelength division multiplexing (WDM) techniques, which is used to carry multiple signals on a single optical fiber by using different wavelengths for each signal. DWDM technology is mostly used in the long haul network segment, while WDM as well as CWDM technology are used to help carrier companies to maximize their network capacity in the access, metro, and regional network segments.
For optical transmission, certain types of lasers such as directly modulated lasers (DMLs) or electro-absorption modulated lasers (EMLs) are used for a long distance of 2 km to 80 km. Generally, DMLs use distributed feedback structure with a diffraction grating in the waveguide for direct modulation, and are often used for relatively lower speeds, less than 25 Gb/s, and shorter reaches, 2-10 km in telecommunication and data communication applications because of its large chromatic dispersion, lower frequency response, and low extinction ratio, compared to EMLs. On the other hand, an EML is a laser diode integrated with an electro-absorption modulator in a single chip. In the EML, laser properties are not changed by a process of modulation, and thus the EML is advantageous in applications with higher speeds and longer distances, compared to a DML. That is, an EML is mostly used for higher speeds and longer reaches, i.e., 10-80 km in telecommunications and data communication applications, because of smaller chromatic dispersion with a stable wavelength under high speed operation. A DML may be implemented in a single chip, providing a compact design and low power applications. However, EMLs are more costly than DMLs and thus may not be a cost effective solution for many applications.
The term “dispersion” as used herein mean the phenomenon in which the velocity of an electromagnetic wave depends on the wavelength, that is, the phase velocity varies based on frequencies. As such, the term “dispersion” includes a more commonly known “chromatic dispersion” which is a phenomenon in which the different wavelengths or colors of a light beam arrive at their destination at slightly different times. As a result, the dispersion causes a spreading of on or off light pulse that carry digital information. One of the effects of the dispersion is to cause degradation of received electrical signals, e.g., stretch initial binary pulses of information to smear into another over a distance. The effects of the dispersion lower the bit error rate of the receiving system and limit the transmission distance. For example, the dispersion effects usually limit 1550 nm transmission distances in metro networks, e.g., 80-100 km.
With CWDM optical transceivers, for longer wavelengths, the CWDM optical transceivers require use of an EML to meet a transmission distance requirement since a DML's chirp is high and the DML based transceivers cannot support 20 km in the long wavelength region about 1500 nm. Here, the term “a chirp” includes a residual data-dependent phase modulation accompanying a desired intensity modulation. It is noted that DML's chirp may tend to broaden an optical signal spectrum and thus may lead to signal distortions caused by interactions with chromatic dispersion of fibers. Also, strong dispersive broadening of modulated signals may occur in high data rates, and thus without adequate dispersion compensation, each symbol would be broadened and overlap with a number of neighboring symbols, which results in significant inter-symbol interference and distortion of the detected signals.
As such, fiber dispersion coupled with chirp parameter of the emitting laser, e.g., DMLs, affects the optical transmission system performance. For example, the fiber dispersion causes the propagation pulses to spread and overlap, and the chirp parameter produces a wavelength shift arising from the intensity variations of the DML. As such, there have been lots of efforts to overcome the fiber dispersion issues as the transmission distance becomes longer and the data rate becomes higher when the DML based optical transceivers are used.
There are two existing categories of technology in compensating the fiber dispersion. In a first category, it is attempted to overcome the fiber dispersion in an optical domain since the fiber dispersion occurs as transmitted signals propagate through the optical fiber. Such first category of technology for compensating the fiber dispersion includes dispersion compensating fiber (DCF), fiber Bragg grating (FBG) and optical filter, etc.
In a second category, the dispersion is mitigated in an electrical domain. That is, the compensation is done by processing received electrical signals by an optical receiver. The second category of technology for compensating the fiber dispersion including techniques with complicated implementations of feed forward equalization (FFE) and/or decision feedback equalization (DFE). These compensation techniques are generally all implemented in integrated circuits (ICs) or separate chips, which are not integrated into optical transceivers because of their high power consumption and high costs in the development of an independent integrated circuit (IC) or a chip. As such, in the existing technology, the functions for compensating the fiber dispersion are implemented as part of a Serializer and Deserializer (SERDES), and installed as circuits or ICs in a line card, which is separate from the optical transceivers.
As such, there is still a further need for improved and more efficient technology for reducing or compensating optical dispersion effects on received signals, providing lower power consumption and low costs benefits.